The present invention is in the field of automatic optical inspection techniques, and relates to a method and system for inspecting reticles or masks in a manner to simulate the operation of a specific photolithography tool in which this reticle is to be used.
Photolithography is one of the principal processes in the manufacture of semiconductor devices, and consists of patterning the wafer""s surface in accordance with the circuit design of the semiconductor devices to be produced. More specifically, a circuit design to be fabricated on the wafer is first patterned on a mask or reticle (for simplicity, the terms mask and reticle will be used here interchangeably, although in actuality they refer to somewhat different techniques). The wafer is coated with a photoresist material, and is then placed in a photolithography tool to be exposed to light passing through the reticle to produce a latent image of the reticle on the photoresist material. Thereafter, the exposed photoresist material is developed to produce the image of the mask on the wafer. After the completion of the photolithography process, the uppermost layer of the wafer is etched, a new layer is deposited, and the photolithography and etching operations are started again. In this repetitive manner, a multi-layer semiconductor wafer is produced.
As is well known, photolithography tools utilize a lamp or a laser as a light source, and utilize a relatively high numerical aperture (NA) objective to achieve a relatively high resolution. The optics of such tools are generally designed to produce reduction (negative magnification) of the image of the reticle, e.g., ⅕ onto the wafer. Different models use different NA and magnification combination, as designed by the manufacturer of the tool.
It should be appreciated that in order to obtain operating semiconductor devices, the reticle must be defect free. Moreover, in most modern processes, the reticle is used in a repeated manner to create many dies on the wafer. Thus, any defect on the reticle will be repeated multiple times on the wafer and will cause multiple devices to be defective. Therefore, various reticle inspection tools have been developed and are available commercially. One type of such inspection systems, to which this invention pertains, scans the entire reticle using an illumination spot technique to inspect the reticle for defects. Examples of such systems are provided in U.S. Pat. Nos. 4,926,489, 5,838,433, and 5,563,702, and an example is schematically depicted in FIG. 1.
As shown in FIG. 1, a reticle 10 is placed on an x-y stage 20. A laser 30 produces an illumination beam of a relatively narrow diameter. A scanner 40, e.g., a rotating mirror or an acousto-optic deflector (AOD), is used to scan the beam in one direction, generally referred to as the xe2x80x9cfast scanxe2x80x9d direction. The stage 20 is moved in a direction perpendicular to the fast scan direction in a serpentine manner, so that the entire surface of the reticle is scanned. The scanned beam passes through the dichroic mirror 50 and is focused by objective lens 60 onto the reticle. Light transmitted through the reticle 10 is collected by the objective lens 70 and focused onto light sensor 80, e.g., a photo-multiplier tube (PMT). Reflected light is deflected by the dichroic mirror 50 to be collected by the lens 95 and focused onto the light sensor 90. Shown by a dotted line is an optional optics and tilted mirror assembly that can be used to obtain an interferometer image of the reticle for inspection of phase shift designs (see, e.g., the cited U.S. Pat. No. ""702).
Conceptually, the inspection systems exemplified in FIG. 1 generate a highly magnified image of the reticle. Each pixel in the image corresponds to a sampled illuminated spot on the reticle, and has a grey level corresponding to the amount of light received by the light sensor. This grey level can be either compared to a corresponding pixel from an adjacent die on the reticle, or binarized and compared. to a database or compared to a gray scale image calculated from the database. When a discrepancy above a designated threshold is encountered, the location is identified as suspected of having a defect.
Recent advancements in photolithography technology have introduced another factor which may cause the latent image on the wafer to be defective. Specifically, the reduction in design rules necessitates various measures to counter changes in the latent image caused by the interaction of the light with the design on the reticle. Such interactions are generally referred to as xe2x80x9coptical proximity effectsxe2x80x9d, and result in, for example, comer rounding, a difference between isolated and semi-isolated or dense patterns, a lack of CD linearity, etc. Whilst not being detected as potential defects. in a particular reticle by the conventional inspection system, these effects could produce real defects on the wafer. On the other hand, these effects should not cause the system to issue an alarm if they will not be transferred as defects onto the wafer. Moreover, there is a need to inspect the countermeasures, such as optical proximity correction OPC and phase shift etching on reticles, and test their design and effectiveness.
Conventionally, in designing and evaluating reticles, especially advanced reticles having OPC and phase shift features, one has to create the reticle, expose a wafer using the reticle, and check that the features of the reticle have been transferred to the wafer according to the design. Any variations in the final features from the intended design necessitate modifying the design, creating a new reticle, and exposing a new wafer. Needless to say, such a process is expensive, tedious, and time consuming. In order to short-cut this process, and to assist in design and evaluation of advanced reticles, IBM has recently developed a microscope called the Aerial Image Measurement System (AIMS).
The AIMS system is disclosed, for example, in European Patent Publication No. 0628806, and in the following articles: Richard A. Ferguson et al. xe2x80x9cApplication of an Aerial Image Measurement System to Mask Fabrication and Analysisxe2x80x9d, SPIE Vol. 2087 Photomask Technology and Management (1993) pp. 131-144, and R. Martino et al. xe2x80x9cApplication of the Aerial Image Measurement System (AIMS(trademark)) to the Analysis of Binary Mask Imaging and Resolution Enhancement Techniquesxe2x80x9d, SPIE Vol. 2197 pp. 573-584. The Microscope is available commercially from Carl Zeiss, GmbH of Germany, under the trade name MSM100 (standing for Microlithographhy Simulation Microscope).
Conceptually, rather than obtaining a highly magnified image of the reticle, as is done by inspection systems, the AIMS system emulates a stepper and creates a highly magnified image of the latent image produced by the reticle. Specifically, the operational parameters of illumination and light collection in the AIMS, such as wavelength and NA, can be adjusted by the user to simulate the tool which will be used to expose wafers using the reticle. The illumination is provided in a manner which simulates exposure in a stepper, so that a latent image of the reticle is created. However, rather than placing a wafer at the location of the latent image, a sensor is placed so as to produce an aerial image of the latent image produced by the reticle. Also, rather than providing reduction of the image like a stepper, the AIMS magnifies the latent image to enable easier image acquisition.
The AIMS is basically an engineering tool, which is intended for development and testing of various reticle designs. It is also helpful for checking how OPC and phase shift features would print on the wafer. Additionally, the system can be used to study various defects discoverred by a reticle inspection systems, and test whether those defects would actually print on the wafer. However, the MSM 100 is not intended to be used as a general reticle inspection system, and lacks any of the technology required for rapid inspection of reticles.
U.S. Pat. No. 5,795,688, however, discloses a technique, for using a system such as the MSMI00 to perform an automatic inspection of a photomask. To this end, an aerial image of a portion of the photomask is acquired with the MSMI00, while a so-called xe2x80x9cvirtual stepperxe2x80x9d software algorithm concurrently simulates a similar aerial image considering the operational conditions of a specific stepper of interest, using the reticle pattern data base. The real aerial image is compared to the simulated aerial image, and potential defects on the photomask are located. This technique actually utilizes a so-called die-to-database image processing technique, wherein the database is constituted by the simulated image. Since the image is obtained using the MSMI00, which cannot perform rapid inspection, this technique cannot be used for in-line automatic inspection of reticles progressing on a production line. On the other hand, this technique does not provide reliable results due to limitations of the simulation software. Specifically, many artificial differences between the real aerial image and the simulated aerial image would be falsely flagged as defects.
Accordingly, there is a need in the art for a reticle inspection system which would be capable of xe2x80x9cconventionalxe2x80x9d reticle inspection in conjunction with Aerial image inspection. Moreover, the system would preferably be also capable of detecting particles on the reticle.
The present invention provides the advantages of automatic optical inspection of reticles utilizing the laser spot illumination, incorporating a novel optical inspection method and system simulating the operation of a specific stepper and specific resist.
It is a feature of the present invention that it can be constructed by easily modifying any conventional inspection system utilizing a flying spot scanning of the reticle under inspection.
The present invention utilizes the capabilities of conventional inspection systems to provide inspection using high resolution imaging of the reticle. Additionally, the inventive system is capable of inspecting the reticle using aerial imaging.
According to another embodiment, the above aperture or a second aperture is inserted into the beam""s path to emulate the effects of the photoresist in lithographic process.
According to another embodiment, the shape of the illumination beam is modified from a Gaussian to a flat-top shape.
According to yet another embodiment the system is made to go out of focus or move some optical elements from their previous location to effectively expand the beam on the reticle.
According to one embodiment, a rotating scattering disk is inserted at a plane where the optical beam has a very small instantaneous diameter so that it shall reduce the time and spatial coherence of the beam on the reticle.
According to another embodiment, an aperture is inserted in the illumination channel, so that the numerical aperture of the illumination objective may be desirably adjusted. By utilizing such an aperture which also has the properties of a beamshaper, the profile of the incident beam may be changed from a gaussian to, for example, a flat-top. The collection channel also includes an aperture accommodated in front of the detector for adjusting the numerical aperture of the collection objective. In other words, by appropriately selecting the illumination and collection apertures, coherence of light in the inspection system can be adjusted to that of a selected exposure tool.
There is thus provided according to one aspect of the present invention, a system for automatic optical inspection of a reticle to be used in a selected exposure tool operating with a selected frequency of light and a selected coherence of the light, the system operating in a direct inspection mode and an aerial imaging mode, the system comprising:
a scanning apparatus scanning the reticle with a flying spot of a light beam and producing light components transmitted through the reticle;
an illumination objective positioned in the path of the flying spot and having an inherent numerical aperture for high resolution illumination;
an aperture selectively insertable in the path of the flying spot to modify the effective numerical aperture of the illumination objective during aerial imaging mode;
an aperture in the detection unit receiving at least the transmitted light components and generating data representative thereof.
Preferably, the analysis of the data representative of the at least transmitted light components includes the comparison of data representative of at least some of the successively scanned features on the reticle to each other. This is the so-called xe2x80x9cdie-to-diexe2x80x9d signal processing technique.
The aperture may be an off-axis aperture, e.g., quadrupole. The illumination aperture may comprise beam-shaping properties, being, for example, a diffractive optical element providing a flat-top beam profile. Preferably, the aperture reduces the numerical aperture of the illumination by about a factor of four, thereby simulating the lower numerical aperture of a stepper.
Preferably, the first directing assembly comprises a set of different apertures. Accordingly, the system can resemble the operation of a different stepper of interest by selecting the illumination aperture type.
The second light directing assembly includes a, light collection aperture, which preferably provides the collection numerical aperture of for example 1.2-0.2.
The detection unit comprises at least one detector accommodated in the optical path of light transmitted through the reticle, which is preferably a photomultiplier tube (PMT).
The system may additionally utilize a dark-field inspection. To this end, the detection unit comprises at least one additional detector accommodated so as to collect light scattered from the illuminated spot on the reticle.
Additionally, to speed up the inspection, the system may utilize a so-called multispot scanning technique. For this purpose, the scanning apparatus also includes a beam splitter means for splitting the primary laser beam into at least two beams, thereby providing at least one additional scanning beam. In this case, the detection unit comprises at least one additional detector for receiving light components transmitted through a spot illuminated by the additional beam and a lens to separate the two beams to the two detecting elements.
According to another aspect of the present invention, there is provided a method for the automatic optical inspection of a reticle to be used in a selected stepper operating with a selected frequency of light, a selected coherence of the light, and a selected type of resist, the method comprising the steps of:
(a) scanning the reticle with a flying spot of a laser beam having said selected frequency;
(b) passing the incident laser beam and the transmitted light components through first and second light directing assemblies, respectively, thereby adjusting said selected coherence of the light;
(c) collecting at least the transmitted light components and generating data representative thereof, and
(d) analyzing said data and generating data indicative of defects on the reticle.
More specifically, the present invention is used for inspecting the reticles used for patterning wafers during the photolithography process, and is therefore described below with respect to this application. It should be appreciated that the terms xe2x80x9creticlexe2x80x9d and xe2x80x9cmaskxe2x80x9d are used herein interchangeably.